System information

SONET and OC circuits
The Synchronous Optical Network (SONET) was developed out of a desire to take the
T-carrier system to the next technological level: fiber optics. SONET is based on the
bandwidth of a T3 (44.736 Mbps), with a slight overhead making it 51.84 Mbps. This
is referred to as an OC-1 or STS-1. As Table A-3 shows, all higher-speed OC circuits
are multiples of this base rate.
Table A-3. OC circuits
Carrier Equivalent data bitrate Number of DS-0s Data bitrate
OC-1 1 DS-3 (plus overhead) 672 51.840 Mbps
OC-3 3 DS-3s 2,016 155.520 Mbps
OC-12 12 DS-3s 8,064 622.080 Mbps
OC-48 48 DS-3s 32,256 2488.320 Mbps
OC-192 192 DS-3s 129,024 9953.280 Mbps
SONET was created in an effort to standardize optical circuits, but due to its high cost,
coupled with the value offered by many newer schemes, such as Dense Wave Division
Multiplexing (DWDM), there is some controversy surrounding its future.
Digital Signaling Protocols
As with any circuit, it is not enough for the circuits used in the PSTN to just carry (voice)
data between endpoints. Mechanisms must also be provided to pass information about
the state of the channel between the endpoints. (Disconnect and answer supervision
are two examples of basic signaling that might need to take place; caller ID is an example
of a more complex form of signaling.)
Channel Associated Signaling (CAS)
Also known as robbed-bit signaling, CAS is what you will use to transmit voice on a
T1 when ISDN is not available. Rather than taking advantage of the power of the digital
circuit, CAS simulates analog channels. CAS works by stealing bits from the audio
stream for signaling purposes. Although the effect on audio quality is not really no-
ticeable, the lack of a powerful signaling channel limits your flexibility.
When configuring a CAS T1, the signaling options at each end must match. E&M (Ear
& Mouth or recEive & transMit) signaling is generally preferred, as it offers the best
supervision. Having said that, in an Asterisk environment the most likely reason for
you to use CAS would be for a channel bank, which means you are most likely going
to have to use FXS signaling.
CAS is very rarely used on PSTN circuits anymore, due to the superiority of ISDN-PRI.
One of the limitations of CAS is that it does not allow the dynamic assignment of
channels to different functions. Also, caller ID information (which may not even be
612 | Appendix A: Understanding Telephony